Photodegradable polymeric compositions containing a mixture of an iron salt and sulfur or a dialkyl polysulfide

ABSTRACT

The invention provides a photodegradable composition comprising a polymer of monoethylenically unsaturated hydrocarbon monomer and, as additives, an iron salt in an amount of 10 to 200 mg per kg of polymer and either sulphur or a dialkyl-polysulphide in an amount of 10 to 1000 mg per kg of polymer. The compositions degrade upon prolonged exposure to the atmosphere but are stable under processing conditions.

I United States Patent 11 1 1111 3,882,058

Le Brasseur, nee Nicoud et al. May 6, 1975 PHOTODEGRADABLE POLYMERIC COMPOSITIONS CONTAINING A MIXTURE [56] References Cited OF AN IRON SALT AND SULFUR OR A UNITED STATES PATENTS DIALKYL POLYSULFIDE 2,982,756 5/1961 Mercier 6161. 260/4595 75 Inventors; G i Le Brawn. nee Nicoud 3.143.534 8/1964 Roberts et al. 260 4595 BUHYleSMines, Adrien Nicco 3,299,568 1 1967 Tobolsky et al. 47 9 Beth n both F 3,454,5l0 7/1969 Nev/land et 260/23 3,676,40! 7 1972 Henry 260/63 [73] Assignee: Ethylene Piastique,Courbevoie,

France Primary Examiner-Eugene C. Rzucidlo [22] F1led: Sept. 14, 1973 [57] ABSTRACT [21] PP N03 397,272 The invention provides a photodegradable composition comprising a polymer of monoethylenically unsat- [30] Foreign Appucflion Priority Data urated hydrocarbon monomer and, as additives, an

Se 20 972 France 72 3336! iron salt in an amount of IO to 200 mg per kg of polyp mer and either sulphur or a diaIkyl-polysuiphide in an 52 us. c|..... 260/23 11- 260/45 7 s- 260/45 75 P' mg Per kg Pdyme" The zfiolDl's compositions degrade upon prolonged exposure to the [51] In Cl C08 45/36 Cosf 45/46 atmosphere but are stable under processing condi- 58 Field 61 Search... 26o/D1o. 43. 45.75 R, 23 H,

3 Claims, No Drawings PHOTODEGRADABLE POLYMERIC COMPOSITIONS CONTAINING A MIXTURE OF AN IRON SALT AND SULFUR OR A DIALKYL POLYSULFIDE The present invention relates to compositions which undergo controlled photodegradation.

There is considerable pollution caused by discarded plastics articles, such as packagings, mugs and bottles, and by polymer films used in agriculture for mulching or for protecting crops. The articles are generally stable to aging under natural conditions and do not break down even upon prolonged aging. It would be desirable if such articles could be fabricated from photodegradable compositions in order that they could break down upon exposure to atmospheric conditions into little pieces which could readily be incorporated into the earth, for example by hoeing or ploughing.

Various photodegradable systems have been described. Thus attempts have been made to prepare photodegradable copolymers of a monoethylenically unsaturated hydrocarbon monomer with at least one other copolymerisable monomer. These copolymers are useful but their production poses technical and economic problems. Photodegradable compositions have been prepared by incorporating photosensitising organic compounds, such as benzophenone, in polymers of monoethylenically unsaturated hydrocarbon monomers. These compositions suffer the disadvantages that the photosensitising compounds are difficult to incorporate in the polymer, resulting in sweating, and that the compositions tend to degrade too soon. Organic salts, which may be stable or labile to heat, of certain metals such as iron have also been used as photosensitising agents. These salts have a known photodegrading effect but they cause rapid thermal oxidation and degradation of the polymers during processing.

lt should be noted, furthermore, that ions of metals such as iron activate the oxidation of numerous organic compounds and have a catalytic effect on the thermal oxidation of polyethylene. The addition of antioxidants, at the same time as iron salts, causes, at the temperatures for processing polyethylene l60230C.), a rapid reaction between the two additives and the formation of more or less stable complex compounds. These complexes are capable of protecting polyethylenes, and more generally polyolefins, at the start of the photo-oxidation reactions, and then of causing rapid degradation of the polymer. However, the chemical reaction mentioned above, which results in the formation of the complexes, leads to results which vary according to the methods of incorporating the additives in the polymer, and thus to final products which have varying photodegradation properties. Finally, it must be mentioned that the rates of photodegradation which can be achieved with such complexes are limited.

The present invention provides compositions which can be rapidly photodegraded but which are stable within the temperature range at which they are to be processed.

The present invention provides a photodegradable composition which comprises (i) a polymer ofa monoethylenically unsaturated hydrocarbon monomer, (ii) to 200 mg of at least one iron salt per kg of the polymer and (iii) 10 to 1,000 mg of sulphur or a dialkyl polysulphide per kg of the polymer.

The iron salt (ii) preferably is an organic salt such as ferric naphthenate, ferric salicylate or a ferric salt of a linear or branched, saturated or unsaturated aliphatic carboxylic acid of 2 to 22 carbon atoms, such as ferric formate, ferric acetate and ferric stearate. The additive (iii) is either sulphur itself (flowers of sulphur) or a dialkyl polysulphide, preferably of the formula R-S,,-R in which R is an alkyl group of 1 to 20 carbon atoms and n is an integer between 4 and 10. Mixtures of sulphur and one or more dialkyl polysulphide or mixtures of dialkyl polysulphides can be used also.

The polymers which can be rendered photodegradable according to the present invention are polymers of monoethylenically unsaturated hydrocarbon monomers such as ethylene, propylene, but-l-ene and styrene. The polymers can be homopolymers, such as high and low density polyethylenes, polypropylene, polybutene and polystyrene, or copolymers of the hydrocarbon monomers with one another or with other copolymerisable monomers such as vinyl acetate, vinyl chloride, methyl vinyl ether, ethyl vinyl ether, acrylonitrile, acrylic esters, carbon monoxide and sulphur dioxide. Ternary copolymers of the acrylonitrilelbutadiene/styrene type can also be used. The copolymers which can be used according to the invention can be random, block or graft copolymers.

lt has been found that the additives (ii) and (iii) used according to the present invention do not react with one another at the temperatures, generally below about 200C, used for converting the polymers into finished or semi-finished products. This absence of interaction makes it possible to prepare products with reproducible properties. When it is necessary to heat the compositions of the invention to temperatures above 200C, it will be noted that the reaction products formed have only a slight photo-oxidising action and thus do not contribute significantly to the photodegradation of the polymer. The rate of photodegradation of products produced from the compositions of the invention can be controlled by changing the relative amounts of additives added, as demonstrated in the Examples and Comparative Examples.

It is possible to add other known additives, such as anti-oxidants and slip agents to the polymer used. lt should be noted, however, that if the polymers contain conventional anti-oxidants, such as phenolic derivatives, a certain amount of complexing of the iron salt (ii) with the anti-oxidants can take place. Under these conditions, in order to achieve the same rate of photodegradation, everything else being equal, it will be necessary to use a larger amount of iron salt (ii) with a polymer containing such anti-oxidants than with a polymer which does not contain any such anti-oxidants.

The compositions of the invention can be prepared in a conventional manner by mixing intimately the polymer (i) and the additives (ii) and (iii). This mixing can be effected, for example by kneading or calendering, at a temperature such that the additives do not decompose. Although the order in which the additives are introduced into the polymer is not controlling, the additive (iii) preferably is introduced first, followed by the iron salt (ii).

The following Examples and Comparative Examples serve to illustrate the invention. In the Examples and Comparative Examples the following methods were used:

The polymer and the additives are blended at the chosen temperature for a definite period in a small laboratory Brabender mixer (polyethylene: 20 mins. at 120; polypropylene: 2 mins. at 190C.)

These results show that the rate of photodegradation of the polyethylene increases with the iron content of the polymer.

However, the stability of these products, during their T cfmposifion obtained is ground in 3 Thomas 5 processing, is not good: Even under the very mild con- W1ley grinder and then moulded 1n the form of a sheet ditions chosen for the preparation of the samples 500 p. thick. o

. (blending at l20 C., moulding at 190C), a marked Dumb-bell shapes are cut out of the sheet, according i change in the Initial mechamcal propertles is noted relto the standard speclficauon for the determination of ative to those of the polyethylene without additives, mechan1calpropert1es(NF T 034). h h h h l k The mechanical properties tested, namely the elonw moves, t at t ere taneous cross m mg gation at break (EB%) expressed as of the length of and degradauon' the original test piece, and the breaking load (BL COMPARATIVE EXAMPLE 2 kglcm expressed 1n kg per square centimetre, are l the determined Effect of the addition of sulphur (1n the form of flow- The dumb-bell shapes are irradiated at 50C. for a ers of p s alone. t a low d nsity polyethyldefinite period in a weather-O-meter (WO) of the enc having a melt index of2 and containing 40 ppm of ATLAS 6000 type. In this apparatus, the composition a conventional anti-oxidant: of the radiation used (xenon lamps with a filter) is very V i m unt of fl wer of sulphur were added to simllal' to of radiation from the the polyethylene; the mechanical properties of the sam- After irradiation, the elongation at break of the pics are given in m dumb-bell shapes 1s measured again and is compared with the measurement made for the same sample be- TABLE fore irradiation, this measurement being taken as being equal 1 l()() Test No. Sulphur in ppm B L kg/cm E B The results given in the Tables are the averages of the results obtamed in at least four measurements. 5 50 155 660 6 100 177 720 COMPARATIVE EXAMPLE 1 7 300 80 730 Effect of the addition of an iron salt, used alone, to a low density polyethylene having a melt index of 2 and containing ppm f a conventional p id The various samples were then 1rrad|ated in a weath- Varying amounts of ferric stearate were added to the B t for a y g p r ds; e l ngati ns at polyethylene; the mechanical properties of the samples break, after irradiation, are given in Table 2b, based on are given in Table la. 35 lOO for the non-irradiated product).

TABLE 2b Lest Time in hours 170 250 320 450 550 Sulphur content ppm TABLE la The results obtained show that:

For 100 and 300 ppm of sulphur, a slight crosslinking Test Ferric stearate Iron E B B L k /cm 50 takes place in the polyethylene; and the effect of the 8 ppm sulphur on the photochemical degradation is not very 1 0 0 680 marked; the sulphur seems rather to protect the polyl g 53 38 {g} mer and does not cause appreciable degradation.

The various samples were then irradiated in a weath- Effect of the simultaneous addition of sulphur and er-O-meter for different periods; the elongations at ferric stearate to a low density polyethylene having a break after irradiation are given in Table [b (based on melt index of 2 and containing 40 ppm of a usual antil00 for the non-irradiated product). oxidant:

TABLE lb Test Time 50 100 250 320 450 550 No. hours Iron COlllGlll Varying amounts of flowers of sulphur and ferric stearate were added to the polyethylene; the properties of the mixtures, before irradiation, are given in Table 30 together with those of certain mixtures from Comparative Examples I and 2:

Various of the samples were then irradiated in a weather-O-meter for varying periods; the elongations at break, after irradiation, are given in Table 3b (based on I for the non-irradiated product).

It is to be noted that:

For all the Tests 8 to 13, the sulphur and the ferric stearate are added together to fused resin in the mixer; and in Test l4, the sulphur was added first followed, mins. later, by the stearate, without markedly changing The overall duration of the mixing is 20 mins. at 120C. for all the tests.

Similar results were obtained by replacing the sulphur with dialkyl polysulphides such as di-(tertiary dodecyl)polysulphide or di-(tertiary dodecyl)disulphide.

We claim:

1. A photodegradable composition which comprises an admixture of (i) a polymer of a monoethylenically unsaturated hydrocarbon monomer, (ii) l0 to 200 mg per kg of the polymer of at least one iron salt selected from the group consisting of ferric naphthenate, a ferric salt of an aliphatic carboxylic acid of 2 to 22 carbon atoms and ferric salicylate and (iii) 10 to L000 mg of an additive selected from the group consisting of sulphur and dialkyl polysulphides per kg of the polymer.

2. A composition according to claim 1, wherein the polymer (i) is selected from the group consisting of homopolymers and copolymers of a hydrocarbon monomer selected from the group consisting of ethylene, propylene, but-l-ene and styrene.

3. A composition according to claim 1, wherein the dialkyl polysulphide is of the formula RS,.-R in which R is an alkyl group of 1 to 20 carbon atoms and n is an integer between 4 and 10.

the results.

TABLE 3b TcSl N0v Iron Time 50 I00 I70 250 320 400 550 content hours Sulphur content 2 l0 0 88 85 42 27 12 IO 211 I0 10 95 92 79 I5 5 2b I0 20 I02 I00 I05 73 39 6 I5 10 30 HO Ill H6 89 54 l0 15a I0 I08 I15 92 I05 44 I3 3 I00 0 86 53 8 l3 3 3 8 I00 I0 94 41 l l 7 5 9 I00 50 89 48 I0 I0 10 I00 I00 9| 82 I I 8 lOa I00 200 I03 90 69 23 l l I 1 I00 300 I I l 95 97 I7 I2 I00 500 H6 I03 H2 H2 36 I3 I00 5,000 9I 89 87 lol 77 

1. A PHOTODEGRADABLE COMPOSITION WHICH COMPRISES AN ADMIXTURE OF (I) A POLYMER OF A MONOETHYLENICALLY UNSATURATED HYDROCARBON MONOMER, (II) 10 TO 200 MG PER KG OF THE POLYMER OF AT LEAST ONE IRON SALT SELECTED FROM THE GROUP CONSISTING OF FERRIC NAPHTHENATE, A FERRIC SALT OF AN ALIPHATIC CARBOXYLIC ACID OF 2 TO 22 CARBON ATOMS AND FERRIC SALICYLATE AND (III) 10 TO 1,000 MG OF AN ADDITIVE SELECTED FROM THE GROUP CONSISTING OF SULPHUR AND DIALKYL POLYSULPHIDES PER KG OF THE POLYMER.
 2. A composition according to claim 1, wherein the polymer (i) is selected from the group consisting of homopolymers and copolymers of a hydrocarbon monomer selected from the group consisting of ethylene, propylene, but-l-ene and styrene.
 3. A composition according to claim 1, wherein the dialkyl polysulphide is of the formula R-Sn-R in which R is an alkyl group of 1 to 20 carbon atoms and n is an integer between 4 and
 10. 